1. Field of the Invention
The present invention relates to a touch signal probe and a signal processing apparatus and a signal processing method of the touch signal probe, the touch signal probe being attached to a surface texture measuring machine such as a form measuring instrument or a CMM (Coordinate Measuring Machine). More particularly, the present invention relates to a touch signal probe, and a signal processing apparatus and a signal processing method of the touch signal probe which comprises a fixed member, a movable member, a reseat position system for allowing displacement of the movable member relative to the fixed member when an external force acts on the movable member and precisely restoring the movable member to a still position when the force acting on the movable member disappears, and deformation detection elements attached to a stylus.
2. Description of the Related Art
With the CMM, a touch signal probe is widely used for measuring a surface texture of a work by detecting contact between the probe and the work. With the CMM using such a touch signal probe, a contact element of the probe that can make a relative move in a three-dimensional direction is brought into contact with a work placed on a stage. The coordinate values of axes (axes in the three-dimensional direction) of the contact element are read with the instant at which the contact element comes in contact with the work as an electric trigger. Then, the dimensions and the shape of the work are found based on the coordinate values. Thus, the contact state between the probe and the work can be used as an electric touch signal to detect the position.
FIG. 8 shows a touch signal probe in a related art. In FIG. 8, the touch signal probe in the related art comprises a stylus 1, a movable member 2, posts 3, a housing 4, a fixed member 5, hardballs (steel balls) 6, bias means 7, and a contact element 8. The stylus 1 is fixed to the movable member 2. The stylus 1 is provided at the tip with the spherical contact element 8. In an outer circumference face of the movable member 2, three posts 3 are provided radially with a 120-degree spacing within a plane at the right angle to the axis of the stylus 1 and with the axis of the stylus 1 as the center. On the other hand, three pairs of hard balls 6 are fixed to the fixed member 5 at the positions corresponding to the posts 3 of the movable member 2. The posts 3 and the hard balls 6 form reseat position elements for uniquely defining the relative position between the fixed member 5 and the movable member 2.
In such a configuration, the movable member 2 is pressed against the fixed member 5 by the action of a bias force F of a bias means 7 and is forcibly brought into contact with the fixed member 5 through reseat position members. The movable member 2 is standing still at six contact points with the fixed member 5 in the state that the press force from the work is applied to the contact element 8 provided at the tip of the stylus 1. This means that each post 3 of the movable member 2 is standing still at two contact points with two hard balls 6 (at six contact points as a whole). Therefore, this is called a six-point contact type reseat position system. The six contact points are connected in series electrically as switch. The contact element 8 comes in contact with the work W and the movable member 2 performs relief operation, so that an out-of-contact condition occurs at any of the six contact points and thus a touch signal can be produced.
In such a six-point contact type reseat position system, the restoration position after the movable member 2 performs relief operation is defined uniquely. That is, assuming that the stylus 1 is displaced in each contact point direction in parallel with the axial direction of the still position of the stylus 1 while the stylus 1 is held in contact with a movable member side reseat position member (posts 3) and a fixed member side reseat position member (hard balls 6), each locus drawn by the tip of the stylus 1 crosses the axis at the still position of the stylus 1. According to such a configuration, at the restoration operation time after the movable member 2 performs relief operation by the press force from the work W, the contact with each contact point is only recovered by the bias force F and the stylus 1 is restored to the unique still position and the still position of the stylus 1 can be held constant.
With the six-point contact type reseat position system, the position of the movable member relative to the fixed member is determined uniquely by contact at six points and thus vibration-resistant rigidity in a reseat position state is high. If press force is applied from any direction, the six-point contact type reseat position system has high restoration accuracy when viewed in comparatively rough order in 10-xcexcm units, for example.
The touch signal probe uses an out-of-contact condition of each contact point in the reseat position system as a touch signal. A touch signal is not output at the instant at which the contact element 8 actually comes in contact with the work W. Signal output is delayed as much as deformation (or distortion) of the stylus 1. If the stylus 1 is long, the tendency becomes noticeable, leading to a bottleneck in high accuracy of measurement.
To deal with this problem, a touch signal probe for detecting deformation of a stylus and producing a touch signal, as shown in FIGS. 9A and 9B is proposed (Japanese Patent Unexamined Publication No. Hei. 10-288502).
In FIG. 9A, a stylus 22 has, at one end, a contact element 24 for coming in contact with a work. Four piezoelectric elements 121 to 124 are attached to a roughly columnar part at an opposite end of the stylus 22.
Each of piezoelectric element support parts 101C and 101D is a flange-like rectangular parallelepiped which is square in cross section orthogonal to the stylus axis. The piezoelectric elements 121 to 124 are fixedly secured to full sides of both rectangular parallelepipeds with an adhesive, etc., so as straddle both rectangular parallelepipeds. According to such a structure, the piezoelectric elements are used as deformation detection elements of the stylus to detect deformation (distortion) of the stylus 22 when the contact element 24 comes in contact with a work W for producing a touch signal.
The detection accuracy of the touch signal probe comprising such a deformation detection type stylus can be 1 xcexcm or higher performance.
Then, if an attempt is made to attach the deformation detection type stylus 22 to the movable member 2 in place of the stylus 1 and use the above-described six-point contact type reseat position system in combination, the reseat position accuracy of the reseat position system becomes insufficient. That is, when the movable member performs relief operation, the contact element of the movable member is pushed into the work and causes relative displacement to the fixed member.
With the six-point contact type reseat position system, from the viewpoint of higher accuracy, for example, 1 xcexcm or less, at the time of the restoration operation after contact, the relative displacement between the movable member and the fixed member causes an error in restoration position (reseat position shift error) to occur.
That is, if the contact element 8 comes in contact with the work W in the orthogonal direction to the axis of the stylus 1 and is pushed into the work W, the stylus 1 and the movable member 2 are inclined and the hard balls 6 and the post 3 are brought out of contact. At this time, drag almost in opposite directions occurs between the movable member 2 and the fixed member and a slight shift in the orthogonal direction to the axis of the stylus occurs in the movable member 2. After this, if the work W and the contact element 8 are brought out of contact with each other, the movable member 2 performs restoration operation by the bias force F, but a restoration position shift (reseat position shift) occurs between the movable member 2 and the stylus 1 because of the above-mentioned shift. The restoration position shift directly affects the measurement accuracy of the probe.
As a reseat position system for correcting such a restoration position shift after the restoration operation, a reseat position system shown in FIG. 10 is proposed (Japanese Patent Unexamined Publication No. Hei. 10-96618). With the reseat position system, piezoelectric elements, etc., are used to manage the direction of the frictional force acting on the contact point between a movable member and a fixed member of the reseat position system, thereby correcting a reseat position shift.
The reseat position system comprises a fixed member 11, a movable member 21, and bias force generation means (not shown). The bias force generation means allows displacement of the movable member 21 relative to the fixed member 11 when an external force acts on the movable member 21 and restores the movable member 21 to a still position when the force acting on the movable member 21 disappears. A stylus 22 having a spherical contact element 24 for coming in contact with a work is provided to the movable member 21. The movable member 21 has, on the outer circumference face, three posts 23 for coming in contact with the fixed member 11 radially with a 120-degree spacing in the orthogonal direction to the axis of the stylus 22.
The fixed member 11 is fixed at the center to a probe housing (not shown). The fixed member 11 has three arms 12 extending radially with a 120-degree spacing with the axis of the stylus 22 as the center. Two hard balls 13 are placed on the top face of the tip of each arm 12. In each arm 12, a piezoelectric element 14 as a displacement generation mechanism is placed expandably in the center part of the fixed member 11 inner from the hard balls 13 roughly along the radial direction relative to the axis of the stylus 22. When a voltage is applied to each piezoelectric element 14, the piezoelectric elements 14 are displaced in synchronization with each other and the hard balls 13 are displaced in the roughly radial direction with the axis of the stylus 22 as the center. The term xe2x80x9cdisplacementxe2x80x9d mentioned here is static displacement and displacement is given gradually by the piezoelectric element. As they are displaced, the directions of the frictional forces at the contact points between the posts 23 and the hard balls 13 are made uniform and the reseat position is adjusted so as to restore to the still position by the bias force.
However, the deformation detection type stylus, which has extremely high detection accuracy, also reacts with various types of noise and outputs a touch signal; the application of the deformation detection type stylus is extremely limited.
That is, to use the deformation detection type stylus and a measuring machine in combination, various vibrations occurring in the measuring machine itself become noise sources. For example, if move operation of each axis of a CMM is performed, a comparatively large vibration occurs in the vicinity of the resonance frequency of each axis structure. An air bearing often used with a precision measuring machine may produce a vibration depending on the structure of an air pad. With motor driving, the carrier frequency of a DC motor undergoing Pulse Width Modulation (PWM) can also become a noise source. Further, noise may also be produced by conversation in a loud voice or walking of a human being in the proximity of the touch signal probe.
The reseat position accuracy of the improved reseat position system is not necessarily sufficient for using the reseat position system and the deformation detection type stylus having extremely high detection accuracy in combination.
It is an object of the invention to provide a touch signal probe, and a signal processing apparatus and a signal processing method the touch signal probe for generating a touch signal reliably with high accuracy without receiving the effect of a noise source.
In order to achieve the above-mentioned object, according to the invention, there is provided a signal processing apparatus of a touch signal probe used for a machine capable of measuring coordinate values of the touch signal probe, the touch signal probe having a fixed member, a movable member to which a stylus comprising a contact element and a deformation detection element is attached, a fixed member side reseat position element being placed on the fixed member, a movable member side reseat position element being placed on the movable member for coming in contact with the fixed member side reseat position element, and a bias member for allowing displacement of the movable member relative to the fixed member when an external force acts on the stylus and restoring the movable member to a still position by a bias force when the external force acting on the stylus disappears, the signal processing apparatus comprising:
a drive circuit for outputting a drive signal for causing the movable member side reseat position element to make a relative move to the fixed member side reseat position element;
a deformation touch signal processing circuit for generating a deformation touch signal from the deformation detection element;
a contact touch signal processing circuit using the fixed member side and movable member side reseat position elements as make-and-break electric contacts to generate a contact touch signal from the make-and-break electric contacts; and
a latch circuit for inputting the coordinate values every instant at which the deformation touch signal is output and storing the coordinate values as the most recent coordinate values for update and when the contact touch signal is output, outputting the most recent coordinate values as detected coordinate values.
In the above-mentioned processing apparatus, it is preferable that the deformation touch signal processing circuit includes a high-pass filter circuit and a low-pass filter circuit. The high-pass filter circuit may have a cutoff frequency ranging from 3 kHz to 10 kHz. The low-pass filter circuit may have a cutoff frequency ranging from 50 kHz to 200 kHz. The deformation touch signal processing circuit may include an amplification circuit wherein gain switching is possible. The he deformation touch signal processing circuit may include a comparison circuit.
Further in the above-mentioned processing apparatus, it is preferable that the deformation touch signal processing circuit includes a monostable multivibrator started by output of the comparison circuit and converts a high-frequency signal output from the deformation detection element by the monostable multivibrator into a digital low-frequency signal. The amplification circuit may switch the gain by a gain signal provided from the machine. The comparison circuit may switch a threshold level by a threshold level signal provided from the machine. The drive circuit may cause the movable member side reseat position element to make a relative move to the fixed member side reseat position element on the basis of a reseat position correcting signal provided from the machine.
In order to achieve the above-mentioned object of the invention, there is also provided a touch signal probe comprising:
a fixed member;
a movable member to which a stylus comprising a contact element and a deformation detection element is attached;
a fixed member side reseat position element being placed on the fixed member;
a movable member side reseat position element being placed on the movable member for coming in contact with the fixed member side reseat position element;
A bias member for allowing displacement of the movable member relative to the fixed member when an external force acts on the stylus and restoring the movable member to a still position by a bias force when the external force acting on the stylus disappears:
a drive member for causing the movable member side reseat position element to make a relative move to the fixed member side reseat position element;
a deformation touch signal processing circuit for generating a deformation touch signal from the deformation detection element;
a contact touch signal processing circuit using the fixed member side and movable member side reseat position elements as make-and-break electric contacts to generate a contact touch signal from the make-and-break electric contacts; and
a signal processing unit having a latch circuit for inputting coordinate values of the touch signal probe every instant at which the deformation touch signal is output and storing the coordinate values as the most recent coordinate values for update and when the contact touch signal is output, outputting the most recent coordinate values as detected coordinate values.
In the above-mentioned touch signal probe, it is preferable that the movable member side reseat position element comes in contact with the fixed member side reseat position element at each contact point with two at three apart places from each other. The drive member may be a contact point displacement member for changing the contact point on the fixed member side and the contact point on the movable member side relatively at least a predetermined distance. The drive member may cause relative vibration only for a given time while holding contact between the contact points on the fixed member side and the movable member side after the external force acting on the movable member disappears.
Further, in the above-mentioned touch signal probe, it is preferable that a diameter of the bias area containing a point biasing the movable member by the bias member is 20% or less of a diameter of a kinematic circle containing the contact points on the circumference with the axis of the stylus as the center and is roughly equal to or more than a pinpoint. A center of the bias area roughly may match a barycentric position of the movable member. A center of the bias area roughly may match a center of the kinematic circle. The bias member may include a helical spring and a length of the helical spring is about one time or more and 2.5 times or less a diameter of the helical spring.
Moreover, in order to achieve the above-mentioned object of the invention, there is provided a signal processing method of a touch signal probe for use with a machine capable of measuring coordinate values of the touch signal probe, the touch signal probe having a fixed member, a movable member to which a stylus comprising a contact element and a deformation detection element is attached, a fixed member side reseat position element being placed on the fixed member, and a movable member side reseat position element being placed on the movable member for coming in contact with the fixed member side reseat position element, a drive member for causing the movable member side reseat position element to make a relative move to the fixed member side reseat position element, an bias member for allowing displacement of the movable member relative to the fixed member when an external force acts on the stylus and restoring the movable member to a still position by a bias force when the external force acting on the stylus disappears, a deformation touch signal processing circuit including an operation circuit for enabling an operation condition to be switched, for generating a deformation touch signal from the deformation detection element, and a contact touch signal processing circuit using the fixed member side and movable member side reseat position elements as make-and-break electric contacts to generate a contact touch signal from the make-and-break electric contacts, the signal processing method comprising:
positioning the touch signal probe before a measurement point of a work;
driving the drive member for making a reseat position correction;
switching the operation condition of the operational circuit for enhancing sensitivity;
feeding the touch signal probe into the measurement point for measuring;
when the deformation touch signal is input, inputting the coordinate values and storing the coordinate values as the most recent coordinate values for update;
when the contact touch signal is input, outputting the most recent coordinate values as detected coordinate values; and
switching the operation condition of the operational circuit for lowering the sensitivity.